BAGHDAD, Iraq (CNN) -- A U.S. soldier fired on his fellow troops at a counseling center at a base outside Baghdad, Iraq, on Monday, U.S. officials said, killing five people in the worst such attack of the six-year-old Iraq war.

The shooting occurred at 2 p.m. at a stress clinic at Camp Liberty, near Baghdad International Airport, two senior defense officials said. Though initial reports indicated the attacker was killed in the incident, the U.S. command in Baghdad said late Monday a suspect in the killings was in custody.

Neither the suspect nor any of the victims had been identified, but a defense official with access to the latest reports on the incident told CNN that the suspect had been a patient at the treatment center.

"Any time we lose one of our own, it affects us all," Col. John Robinson, a U.S. military spokesman, said in